1. Field of the Invention
The present invention relates to glass chopped strands (hereinafter referred to as CS) to be used for a molded product of BMC containing an unsaturated polyester as the main component of the matrix resin, and a molded product of unsaturated polyester resin BMC employing them.
2. Discussion of Background
Glass fibers are widely used as reinforcing fibers for the production of FRP. For industrial purposes, glass fibers are used in the form of glass fiber strands having a number of glass fibers withdrawn from bushing bundled, in many cases. It is common to have a sizing agent comprising a silane coupling agent and a film-forming agent, applied thereto to prevent thread breakage or fuzzing during their handling or production process (process for production of strands) thereby to improve the operation efficiency and to improve the affinity with the resin. The glass fiber strands thus produced, are dried and used as resin-reinforcing fibers. And, a bulk molding compound (BMC) having glass fibers dispersed in a resin, obtained by uniformly kneading cut products of strands (chopped strands, hereinafter referred to simply as CS) with a resin, is commonly used as a molding material for producing FRP. This molded product of BMC is widely used for various applications, because of merits such as the strength, heat resistance, dimensional stability, etc.
Among them, particularly for a molded product to be used for a reflecting mirror of a head lamp to be mounted on e.g. automobiles, it is common to employ a thermosetting resin, since it is necessary to be durable against a high heat generated from the lamp, and to use a molded product of unsaturated polyester resin BMC employing an unsaturated polyester as the main component of the matrix resin.
As such a molded product of unsaturated polyester resin BMC, for example, JP-A-5-293823 discloses, as a molded product of BMC having high strength while maintaining excellent characteristics of a molded product prepared by the BMC molding method, a molded product of BMC wherein glass chopped strands obtained by cutting a glass strand which is obtained by bundling a group of glass filaments having a sizing agent coated on the surface, are used as a reinforcing material, wherein the number of glass filaments constituting non-fibrillated glass chopped strands is at least 0.1% based on the total number of glass filaments, and wherein the solubility in styrene of the sizing agent is preferably at most 40%, and the time for impregnation of the unsaturated polyester resin is within 10 minutes.
However, when the molded product of BMC is used for e.g. the above-mentioned reflecting mirror for lamps, a primer is coated on the surface of the molded product of BMC after molding and then metal vapor deposition is applied as a reflecting film. Therefore, after coating a primer on the surface of the molded product, leveling and heat curing are required, whereby the molded product is exposed to a high temperature of at least 100xc2x0 C. Accordingly, it is required that excellent smoothness is maintained even at such a high temperature.
However, if the molded product of BMC is heated after coating the primer, it has sometimes happened that due to the temperature increase by heating, air bubbles internally present in the molded product expand and migrate to the surface of the molded product, thereby to form defects on the appearance of the surface. Namely, when air bubbles are formed at the surface of the molded product, the primer coating film is pushed up by such bubbles to form bubble-like defects, thus impairing the outer appearance and leading to a decrease in the yield in the vapor deposition process.
Such air bubbles are attributable to air contained between CS and the matrix resin, and they are believed to be formed by the following mechanism depending upon the degree of the impregnation property of the matrix resin to CS or the nature of the sizing agent to bundle CS.
Firstly, in a case where the sizing component for CS is stiff, the matrix resin will not sufficiently penetrate into the interior of CS, whereby wettability between the filaments and the matrix resin can not adequately be secured in the interior of CS. Accordingly, an air layer will be present at the interface between the filaments and the matrix resin and will form air bubbles when subjected to heat treatment.
As such a stiff sizing agent, a urethane resin may, for example, be mentioned. In a case where the matrix resin is an unsaturated polyester resin, a styrene monomer is usually employed as a typical crosslinking agent, but a urethane resin is relatively hardly soluble in this styrene monomer. Accordingly, the urethane resin tends to be stiff as a sizing agent and thus tends to include air bubbles.
Accordingly, with the molded product of BMC as disclosed in the above JP-A-5-293823, although the strength can be improved by adjusting the number of glass filaments constituting non-fibrillated glass chopped strands to be at least 0.1% and adjusting the solubility of the sizing agent in styrene to be at most 40%, there is a problem that the solubility of the sizing agent is insufficient, and accordingly, no adequate wettability is secured between the matrix resin and the filaments in the interior of CS, and inclusion of air bubbles can not be avoided.
On the other hand, if the sizing component for CS is softened in order to improve the wettability (the impregnation property) between CS and the matrix resin, CS will be unbound into monofilaments during kneading with the matrix resin. By this unbounding into monofilaments, the volume of the kneaded product expands, and air will be included among the filaments. The air thus included in the material can not completely be removed at the time of molding and will remain in the resulting molded product thereby to form bubble-like defects.
Further, with respect to a question of strength, if the wettability of CS is improved, the flexural strength will be improved, but there will be a problem that deterioration in the impact strength will be observed.
As such a soft sizing agent, a vinyl acetate resin may, for example, be mentioned. The vinyl acetate resin has a high solubility in a styrene monomer, whereby the wettability between CS and the matrix resin will be improved, but by the above-mentioned phenomenon, air bubbles are still likely to form, and the impact strength will also be deteriorated.
In view of the foregoing, in order to let the BMC molding material containing an unsaturated polyester resin as the main component of the matrix resin include no air bubbles while maintaining the mechanical strength, the sizing agent for CS is required to have a proper wettability with the matrix resin and the respective filaments in CS and have a good integrity free from the volume expansion due to unbinding into monofilaments during the kneading of BMC. However, with conventional sizing agents, it has been impossible to satisfy both requirements simultaneously.
Accordingly, it is an object of the present invention to provide CS having a good integrity and a proper wettability with a matrix resin containing an unsaturated polyester as the main component, so that a molded product of an unsaturated polyester resin BMC is free from air bubbles on the surface of the molded product even at a high temperature and has a property excellent in impact strength, and a molded product of an unsaturated polyester BMC employing such CS.
In order to solve the above problems, the chopped strands of the present invention, are chopped strands having a sizing agent impregnated to glass fiber strands, to be used for a molded product of unsaturated polyester resin BMC, wherein the sizing agent comprises a urethane resin, a vinyl acetate resin and a silane coupling agent, and wherein the mass ratio of the urethane resin to the vinyl acetate resin is from 30:70 to 70:30.
Thus, the sizing agent comprises an urethane resin and a vinyl acetate resin in a prescribed ratio, whereby proper wettability can be obtained between the matrix resin and the respective filaments in CS, and at the same time, the volume expansion by unbinding into monofilaments during kneading of BMC can be prevented. Accordingly, it is possible to prevent formation of air bubbles at the surface of a molded product of BMC obtained by using an unsaturated polyester resin as the matrix, even at a high temperature. Further, the wettability between the matrix resin and the respective filaments in CS will be proper, whereby a molded product of BMC excellent in impact strength, can be obtained.
Further, in a preferred embodiment of the chopped strands of the present invention, the above-mentioned urethane resin comprises an isocyanate component and a polyol component, as the main components, and wherein the isocyanate component contains at least one member selected from the group consisting of dicyclohexylmethane-4,4-diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate and tolylene diisocyanate, and the polyol component contains at least one member selected from the group consisting of a polyester polyol made of a polybutylene adipate or a polyhexamethylene adipate, and a polyether polyol made of a polyoxypropylene polyol or a polyoxybutylene polyol.
By using the urethane resin comprising such an isocyanate component and a polyol component, stiffness of the urethane resin will be reduced, and the urethane resin will be properly swelled by the styrene monomer, whereby proper wettability can be obtained between the matrix resin and the respective filaments, and at the same time, excess fibrillation of CS is prevented, whereby inclusion of air bubbles during kneading can be prevented.
In another preferred embodiment of the chopped strands of the present invention, the above-mentioned vinyl acetate resin contains silanol groups. By such silanol groups, the vinyl acetate resin will be properly swelled, whereby it is possible to prevent unbinding of CS into monofilaments more than necessary by suppression of the solubility in the styrene monomer, and wettability between the matrix resin and the monofilaments will be improved, whereby it is possible to prevent formation of air bubbles in the molded product.
In still another preferred embodiment of the chopped strands of the present invention, the above-mentioned silane coupling agent is an amino type and/or acrylic type, whereby the adhesion between CS and the matrix resin will be improved, and inclusion of air bubbles can effectively be prevented.
In still another preferred embodiment of the chopped strands of the present invention, in a solubility test of the sizing agent, the proportion of the mass of the sizing agent after immersing it in a styrene monomer for 60 minutes, to the mass of the sizing agent before the immersion, is from 3 to 25%. When the solubility is within this range, the wettability of CS will be maintained at a proper level, whereby formation of air bubbles in the molded product can be prevented.
In still another preferred embodiment of the chopped strands of the present invention, in an expansion test of the sizing agent, the proportion of the length of a film of the sizing agent having an aspect ratio of 4:1 after immersing it in a styrene monomer for 60 minutes, to the length in the longitudinal direction of the film before the immersion, is from 1.5 to 2.5 times. If the expansion degree is within this range, it is possible to prevent unbinding of CS into monofilaments more than necessary, and wettability between the matrix resin and the monofilaments will be improved, whereby it is possible to prevent formation of air bubbles in the molded product.
In still another preferred embodiment of the chopped strands of the present invention, in a strand integrity test of the chopped strands containing the sizing agent, the proportion of the volume increase of the chopped strands after immersing and stirring them in acetone for 250 seconds, to the volume before the immersion, is not more than 30%. When the sizing property is within this range, it is possible to prevent unbinding of CS into monofilaments more than necessary, whereby it is possible to prevent formation of air bubbles in the molded product.
In still another preferred embodiment of the chopped strands of the present invention, in a strand integrity test of the chopped strands containing the sizing agent, the mass of fuzz remaining on a sieve having apertures of 4 mm after stirring the chopped strands with steel balls by a paint shaker for 5 minutes, is not more than 1%, based on the amount of the chopped strands introduced. If the sizing property is within this range, the product can be transported without formation of fuzz in the piping, and good handling efficiency can be obtained, and little fibrillation takes place at the initial stage prior to kneading. Accordingly, progress in fibrillation of strands at the time of kneading with the matrix resin, can be controlled, and as a result, it is possible to prevent formation of air bubbles in the molded product.
On the other hand, the molded product of unsaturated polyester resin BMC of the present invention is a molded product of BMC, which is molded by using the above-mentioned chopped strands and which comprises 100 parts by mass of a matrix resin containing an unsaturated polyester resin as the main component, and from 30 to 150 parts by mass of the chopped strands. It is thereby possible to obtain a molded product of BMC which is excellent in mechanical strength and which is free from formation of air bubbles at the surface of the molded product even if it is exposed to a high temperature after the molding.
In a preferred embodiment of the molded product of unsaturated polyester resin BMC of the present invention, in a bulging test of the molded product of unsaturated polyester resin BMC, the number of bulges on the surface of the molded product is zero when the surface of the molded product is coated with a primer, followed by heating for 60 minutes in an atmosphere of 180xc2x0 C. The molded product is thereby free from formation of air bubbles on the surface of the molded product even if exposed to a high temperature after the molding.
In another preferred embodiment of the molded product of unsaturated polyester resin BMC of the present invention, the molded product of an unsaturated polyester resin BMC is obtained by injection molding, and it is to be used for a reflecting mirror for lamps. This molded product is free from formation of air bubbles at the surface of the molded product even when exposed to a high temperature after the molding, and it is particularly suitable as a molded product where a smooth surface for vapor deposition is required, like a reflecting mirror for lamps.
In still another preferred embodiment of the molded product of unsaturated polyester resin BMC of the present invention, the strength of the molded product of unsaturated polyester resin BMC molded by injection molding, is such that in the flexural strength and Izod impact strength test as stipulated in JIS K6911, the flexural strength is at least 65 MPa, and the notched Izod impact strength is at least 60 J/m. It is thereby possible to obtain a molded product which is excellent not only in prevention of formation of air bubbles but also in the mechanical properties such as flexural strength and impact strength. Accordingly, the molded product of the present invention can be used suitably even at a portion where vibration is vigorous, as represented by a reflecting mirror for lamps for e.g. automobiles.